In the furniture industry, and related industries, a wide range of substrates must be adhered, including wood; metal, such as cold-rolled steel and aluminum; fabric; paper; foam; plastic, such as polyvinylchloride, polystyrene, polyethylene, polypropylene, and acrylonitrile butadiene styrene (ABS); fiberglass; and materials used to construct high pressure laminates, for example, for counter tops. In the past, foam and furniture manufacturing has been dominated by one-part contact adhesives dissolved or dispersed in organic solvents, such as chlorinated solvents and low flash point organic solvents. Such one-part contact adhesives are conveniently able to be applied using a single source (i.e., container). Recently, however, there has been a desire to shift from organic, solvent-based adhesive compositions to aqueous-based or aqueous-dispersed adhesive compositions for environmental, health, and safety reasons.
Many contact adhesive applications require that soon after applying the adhesive to the substrates to be contacted and pressing the coated substrates together, the adhesive rapidly bonds to itself. That is, the adhesive undergoes auto adhesion and forms a semi-solid film with sufficient strength to hold the substrates together and resist subsequent forces on the fresh bondline that might cause failure. Adhesives that provide the desired quick strength properties immediately after bonding can be referred to as "fast-set" adhesives. To determine whether an adhesive is a fast-set adhesive, a finger bond test may be utilized. Quick-drying organic solvents have been conventionally utilized as carriers for fast-set contact adhesives since their quick-drying film forming properties facilitate short set times and quick holding together of the bonded parts after their coated surfaces are mated and pressed together.
To comply with consumer demand, it would be desirable to find an aqueous-based contact adhesive that facilitates fast adhesive set times. Attempts to provide them in one-part form, however, have experienced only gradual industry acceptance since they have had longer dry times than conventional organic, solvent-based adhesives, as well as a relatively slow rate of strength build. To overcome such limitations, two-part (i.e., co-sprayed from two separate containers) aqueous-dispersed adhesive systems have been developed that demonstrate high adhesive strength within seconds of spraying. The adhesive composition is one part of the two-part system and an external coagulant, such as citric acid, lactic acid, acetic acid, or zinc sulfate, is typically used as the second part in a predetermined ratio. Such two-part adhesive systems, however, are not entirely satisfactory. The co-spraying equipment is expensive, the equipment requires maintenance, and the ratio of the two parts (the coagulant and the adhesive composition) must be monitored during application. Thus, there is a need for a one-part, aqueous-based, fast-setting, contact adhesive.
Polychloroprene has been used as an aqueous-based adhesive. For example, see Pole et al. (U.S. Pat. No. 4,240,860), where a substantially solvent-free, aqueous-based adhesive composition comprises 2-chloro butadiene. The composition is stabilized with an emulsifier that forms aqueous-insoluble compounds with zinc or cadmium ions. The pH of the adhesive formulation is preferably above 9.5, and more preferably 10.3 to 11.5. Also see, Gerlach ("Polychloroprene-An Evergreen Product for the Formulation of Water Based Contact Adhesives," Advances in Adhesives & Sealants Technology, paper 14), where polychloroprene is suggested as a potentially suitable aqueous-based adhesive. Gerlach mentions that polychloroprene is also often blended with an acrylate dispersion. The acrylate dispersion "provides some initial tack and also a destabilizing effect which accelerates coagulation." (See Gerlach, p. 10).
Polychloroprene has also been used to bond foam. See Simmler et al. (European Patent Office Publication No. 0 624 634 A1), where polychloroprene is used as a copolymer with acrylate acidic ester copolymers. Boric acid is mixed into a one-part, sprayable dispersion of the copolymers. However, one problem that has been found with many of the conventional, one-part, adhesive compositions using polychloroprene is that the compositions have a short shelf life. That is, after extended storage, the compositions coagulate, rendering their application problematic, particularly when spray-coating. The problem is even more pronounced when the compositions are stored at elevated temperatures.
Fast-set contact adhesives have not been obtained without decreasing storage stability. To date, short set times and storage stability have not been satisfactorily obtained within the same adhesive composition in a one-part system. The problem has been that in order to achieve a fast-setting adhesive composition, it is necessary to find some additive or additives to make the resulting adhesive composition sensitized and unstable enough that the colloid polymer system immediately "breaks" on application and forms a high strength film, but yet does not make the composition unstable to the point that it will coagulate during storage and before application. Such additives are sometimes referred to as internal coagulants. Additives used to sensitize anionic-stabilized latex compounds and improve their ability to break upon application are often acidic. For example, Carl, J. C. ("Fluid Properties," Neoprene Latex: Principles of Compounding and Processing, E.I. DuPont de Nemours & Co., p. 18 (1962)) discusses the use of glycine as an amino acid that controllably destabilizes latex compounds containing anionic surfactants. Glycine acts to destabilize the compound by reducing the pH of the system.
Polychloroprene systems often require special considerations. For example, polychloroprene dispersions can be prepared with stabilizers to compatibilize the polychloroprene with its carrier. See Carl, J. C. ("Fluid Properties," Neoprene Latex: Principles of Compounding and Processing, E.I. DuPont de Nemours & Co., pp. 9-11 (1962)) for a discussion of suitable stabilizers. Strong alkali components, such as potassium hydroxide, can also be added to the adhesive system to stabilize the composition during storage (i.e., provide storage stability) by raising the pH of the system. They also neutralize and prevent formation of hydrochloric acid as some of the chlorine on the polychloroprene molecule normally becomes detached from the polymer during long term storage of the latex. Commercially available polychloroprene dispersions commonly have pH values as high as 12 to 13.
It has also been difficult to find satisfactory polychloroprene compositions for use in demanding applications where a fast-setting contact adhesive is necessary. One example of such an application is the formation of pinch bonds during manufacture of foam rubber cushions used in furniture. Pinch bonds, or knife-edge bonds, require even more strength build-up than needed to form a finger bond. Thus, fast-setting contact adhesives are even more important for such demanding applications.
Thus, there is a need for an aqueous-based polychloroprene contact adhesive that is available in one-part form. The contact adhesive must be storage stable, yet capable of fast-setting. It is also desirable that the contact adhesive be capable of developing enough strength for demanding applications within a short period of time.